@tchrist: In fact it appears to be the other way around; the plant is named after the body part. :-O And this is already a euphemism of sorts; according to that same page the Middle English term for the plant was ballockwort (!). If looked at properly one can see why (caution, explicit botanical diagram).
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Nate EldredgeJun 4 '12 at 3:30

Monorchism is a correct word, but cryptorchidism or cryptorchid is probably the more commonly used/heard word since it is the most common birth defect of male genitalia. It is the absence of one or both testes from the scrotum. With cryptorchidism the testicle may be within the abdomen, undescended — but it is absent from the scrotum. A boy or man with one undescended testicle could be called a unilateral cryptorchid. (This condition occurs in veterinary species, as well.)

@CesarGon, They are...they are up in the abdomen (undescended), but that is also true for monorchism.
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JLGJun 3 '12 at 20:58

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I know. The OP seems to be asking for a word denoting someone lacking one testicle, hence my remark.
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CesarGonJun 3 '12 at 23:28

@CesarGon, Lacking one in the scrotum. (Can't believe I'm having a serious discussion about this.) Actually missing a testicle is very rare. It is usually a problem with one or both not descending into the scrotum (as the link I provided shows).
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JLGJun 3 '12 at 23:36

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@DavidSchwartz: No. To get rid of a cancer.
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CesarGonJun 6 '12 at 18:34